xf86-video-ati driver can't work like that. Please check again my previous comment, especially /proc/fb must contain radeondrmfb before the X server is started. radeonfb must be disabled under all circumstances.

If that doesn't work either, leave kernel modesetting disabled for now and downgrade to xf86-video-ati-6.14.6-r1 which is the last version to still support userspace modesetting. Report your problem to https://bugs.freedesktop.org/ together with all relevant logs, so it can be fixed before support for userspace modesetting goes away entirely.